post_exists() doesn’t exist because you forgot to include the file where the function is declared. Looking to https://developer.wordpress.org/reference/functions/post_exists/#source you can see that it’s declared in wp-admin/includes/post.php
Add this to your existing code juste after the require of taxonomy.
require_once ABSPATH . '/wp-admin/includes/post.php';
The best way to achieve what you want is to use wp-api in my opinion !